1. Field of the Invention
The present invention relates to an ultraviolet light beam irradiating apparatus and method for irradiating the surface of a work such as a semiconductor silicon wafer or a substrate for producing a liquid crystal display, with an ultraviolet light beam to treat the surface, i.e., to oxidize and remove an organic material adhering to the surface, to directly oxidize the surface or to modify the surface.
2. Prior Art
A cleaning method using irradiation with an ultraviolet light beam is studied or used for/in part of the cleaning step in the production of a semiconductor integrated circuit or a liquid crystal display. The mainstream of the method is a method using a low-pressure mercury lamp as a light source. The low-pressure mercury lamp is a discharge lamp having spectra mainly in wavelengths of 186 nm and 254 nm.
When an oxygen-existing atmosphere such as air is irradiated with a low-pressure mercury lamp, oxygen molecules are dissociated to oxygen atoms O(3P).
O2+hxcexd(xcex=175xe2x88x92254 nm)xe2x86x92O(3P)+O(3P)
The oxygen atom O(3P) generated above bonds to an ambient oxygen molecule which is not yet dissociated, to generate ozone (O3).
O(3P)+O2xe2x86x92O3
Ozone mainly absorbs light at and around a wavelength of 254 nm and has an absorption band in the wavelength of 220 to 300 nm. Therefore, O3 exposed to a light having a wavelength of 254 nm from a low-pressure mercury lamp absorbs the light and dissociated into an exited-state oxygen atom O(1D) and an oxygen molecule (O2).
O3+hxcexd(xcex=220xe2x88x92300 nm)xe2x86x92O(1D)+O2
While a light at 186 nm has a photon energy of 6.7 eV, typical organic substances have a Cxe2x80x94H bond energy of 3.5 eV and a Cxe2x80x94O bond energy of 3.3 eV which are lower than the photon energy of a light at 186 nm. When an organic compound is irradiated with the above light, therefore, chemical bonds are broken, and further, the organic compound is dissipated and removed by the oxidation strength of ozone and active oxygen species generated by the above processes. When the above method is applied to a clean surface where no organic substances are present, a hydrophilic group such as an OH group or the like is introduced into the surface to render the surface of a work hydrophilic. This procedure is employed before the formation of a film for improving the adhesion of the film.
Meanwhile, for a more efficient method, a cleaning or modification method using an excimer lamp with xenon capable of radiating a light having a shorter wavelength sealed therein has been and is recently studied in place of a low-pressure mercury lamp.
In the excimer lamp, AC high voltage of several kV is applied to a quartz glass tube with xenon gas sealed therein to cause a barrier discharge, and xenon gas molecules are brought into a state of excited bonds, that is, an excimer (excimer state) and a light is emitted during a process from the above state back to their ground state, and the above method uses such a light. In a lamp with xenon gas sealed therein, a radiated light has a wavelength of 172 nm (half-width 14 nm).
The photon energy at 172 nm is 7.2 eV or higher than 6.7 eV of a low-pressure mercury lamp, and the chemical bonds are broken more effectively. Since a radiated light from a xenon excimer lamp contains almost no light at 220 to 300 nm, it neither can decompose ozone nor can generate active oxygen in an excited state. However, when oxygen molecules O2 are irradiated with a light having a wavelength shorter than 175 nm, excited-state oxygen molecules O(1D) can be directly generated.
O2+hxcexd(xe2x88x92175 nm)xe2x86x92O(1D)+O(3P)
As described above, the method using the xenon excimer lamp has many advantages over the method using a low-pressure mercury lamp in that the photon energy is high; excited-state oxygen atoms having high oxidation strength can be directly generated; a light radiated from the xenon excimer lamp has a quasi-single wavelength at 172 nm while light radiated from a low-pressure mercury lamp has spectrum at 186 nm, 254 nm, etc., and the efficiency of taking out (conversion to) vacuum ultraviolet light having a wave length of 200 nm or shorter on the basis of inputted energy (consumption power) is high.
Therefore, the actual cleaning efficiency is also high. The above cleaning or oxidation method using irradiation with ultraviolet light is called a UV/O3 treatment, and the UV/O3 treatment is generally carried out on a work placed in atmosphere. Generally, the efficiency of the UV/O3 treatment increases with an increase in the intensity of UV light with which a work is irradiated, and it also increases with an increase in the concentration of ozone in a treatment atmosphere. In the method using a low-pressure mercury lamp in particular, irradiation at 186 nm is essential for generating ozone. However, light irradiated from a low-pressure mercury lamp has a far higher intensity at 254 nm than it has at 186 nm, and the concentration of ozone generated is not so high. For this reason, it is actual practice to externally introduce ozone generated with an ozone generator into the treatment atmosphere.
In integrated-circuit production process in which a throughput is considered essential, it is considered essential to decrease the length of a processing time, and studies are being made for a method that serves to shorten the UV/O3 treatment time. The most general method is a method of rising the temperature of a substrate. xe2x80x9cTheory and Practice of Ozone Utilizationxe2x80x9d (Realize K. K., issued in 1989, page 309, ISBN:4-947665-29-1) shows influences of the temperature of a work on the efficiency of the UV/O3 treatment. According thereto, when the temperature of a work is 100xc2x0 C., the rate of removal of organic substances is approximately 5 times higher than the rate at 30xc2x0 C.
Further, xe2x80x9cPreoxidation UV Treatment of Silicon Wafersxe2x80x9d (J. Electrochem. Soc, Vol. 134, No.8, 1987, P. 2052, co-written by J. Ruzyllo, G. T. Duranko and A. M. Hoff) describes a method of increasing an oxidation rate by adding water vapor to a treatment atmosphere in the UV/O3 treatment using a low-pressure mercury lamp. It is shown that, in the above method, the oxidation rate is increased by introducing oxygen gas into a bottle containing pure water warmed at 65xc2x0 C. to generate water vapor and adding the water vapor to a treatment atmosphere.
While methods for increasing the oxidation rate are studied in various ways as described above, it is commercially desired to develop an ultraviolet light beam irradiating apparatus and method which can achieve the treatment for a shorter period of time.
It is an object of the present invention to provide an ultraviolet light beam irradiating apparatus and method for irradiating the surface of a work with an ultraviolet light beam to treat the surface, according to which the efficiency of the treatment is improved and the length of the treatment time period is shortened.
It is another object of the present invention to achieve the above object with a relatively simple apparatus constitution.
The ultraviolet light beam irradiating apparatus for irradiating the surface of a work with an ultraviolet light beam to treat the work surface, provided by the present invention, comprises a bed for supporting the work in atmosphere, an ultraviolet light beam source for irradiating the surface of the work with a vacuum ultraviolet light beam having a wavelength of 175 nm or shorter, and inert gas inflow structure for allowing an inert gas, preferably one of nitrogen, helium and argon to flow into a space of the atmosphere on and above the surface of the work.
The above treatment of the work surface includes oxidation and removal of organic substances adhering to the surface, direct oxidation of the surface and modification of the surface.
In the above case, preferably, the above inert gas is mixed with water vapor and the mixture is allowed to flow into the above space.
As shown in the above xe2x80x9cTheory and Practice of Ozone Utilizationxe2x80x9d, oxygen intensely absorbs light having a wavelength of 130 to 175 nm, and an oxygen molecule which has absorbed the light is dissociated directly into an oxygen atom O(3P) in a ground state and oxygen atom O(1D) in an excited state. That is
O2+hxcexd(130xe2x88x92175 nm)xe2x86x92O(1D)+O(3P).
The excited oxygen atom O(1D) generated by the above reaction has higher oxidation power than ozone O3 and serves to carry out cleaning, modification and oxidation efficiently. In the UV/O3 treatment, it is essential to irradiate the work surface with an ultraviolet light beam, since the UV/O3 treatment is based on synergistic effects of oxidation caused by active oxygen species such as O3 and oxygen atoms in an excited state and decomposition of compounds caused by irradiation with an ultraviolet light beam from the light source.
Since, however, oxygen molecules absorb intensely ultraviolet light, the intensity of an ultraviolet light beam decreases with an increase in the distance from the ultraviolet light source. Preferably, therefore, the distance between the ultraviolet light source and the work is as small as possible. In the ultraviolet light beam irradiating apparatus, it is structurally required to provide a certain gap (practically 3 to 5 mm) between the ultraviolet light source and the work.
According to the present invention, an inert gas flows into the above space, whereby the oxygen concentration between the ultraviolet light source and the work surface is decreased, so that the absorption of an ultraviolet light beam is therefore decreased.
Further, an inert gas is mixed with water vapor, and the mixture flows into the above space, whereby OHxe2x88x92 having high oxidation power is generated, so that the effect of UV/O3 treatment is further improved. That is, when irradiated with an ultraviolet light beam, water vapor H2O generates H+ and OHxe2x88x92 according to the following equation.
H2O+hxcexdxe2x86x92H++OHxe2x88x92
OHxe2x88x92 has oxidation power four times as strong as that of O3 and makes it possible to carry out efficient oxidation. Further, since water vapor H2O intensely absorbs a vacuum ultraviolet light, it is mixed with an inert gas in a proper amount ratio so that the substrate surface is irradiated with an ultraviolet light beam.
In the present invention, the above ultraviolet light source can be selected from a xenon excimer lamp (emitted-light wavelength 172 nm), a krypton excimer lamp (emitted-light wavelength 146 nm), an argon excimer lamp (emitted-light wavelength 126 nm) or a fluorine excimer lamp (emitted-light wavelength 157 nm). Further, the above light source may be an excimer lamp using, as an exciting source, any one of dielectric barrier discharge, high-frequency discharge, microwave and electron beam.
In the present invention, further, the above inert gas inflow structure preferably has a constitution comprising a gas passage extending in a direction along the ultraviolet light source; and inert gas supply source for supplying the gas passage with the inert gas; and one or a plurality of discharge ports which communicate with the gas passage and are disposed along the gas passage and which are for discharging into the space the inert gas introduced into the gas passage.
In the above case, preferably, the gas passage and the one or a plurality of the discharge ports are provided on both sides of an irradiation range of the ultraviolet light beam from the ultraviolet light source. The gas passage and the one or plurality of the discharge ports can be formed in a box of the ultraviolet light source or can be formed in a tube placed between the ultraviolet light source and the bed.
Further, preferably, the interval, the size or the form of the one or a plurality of the discharge ports in the inert gas inflow structure are adjusted such that the discharge rate of the inert gas from a plurality of the discharge ports is nearly constant per unit length in the direction along the gas passage.
Further, the apparatus of the present invention may have a constitution comprising a rotary device for rotating the bed such that the work on the bed moves in the irradiation range of the ultraviolet light beam from the ultraviolet light irradiation light source or a moving device for moving the bed such that the work on the bed crosses the irradiation range of the ultraviolet light beam from the ultraviolet light irradiation light source.
The present invention is also concerned with a method of irradiation with an ultraviolet light beam for irradiating the surface of a work with the ultraviolet light beam to treat the surface. The method of the present invention comprises the steps of supporting the work in an atmosphere; allowing an inert gas to flow into a space in the atmosphere on and above the surface of the work; and irradiating the surface of the work with the ultraviolet light beam having a wavelength of 175 nm or shorter.
In the above method, preferably, the insert gas is mixed with water vapor and the mixture is allowed to flow into the space.